In optical communication systems, there has been considerable activity in the development of an optical receiver capable of handling signals transmitted at rates of, for example, 40 Gbps, 100 Gbps or higher per wavelength. Recently, a digital coherent receiver has attracted attention as a receiver in the optical communication systems. The digital coherent receiver may operate as follows. The receiver extracts information about the optical intensity and phase by coherent receiving, and digitizes the extracted information. Then, the receiver demodulates the digitized information by a digital signal processing circuit.
The optical communication systems are configured to employ dispersion compensation including chromatic dispersion. In the digital coherent receiver, chromatic dispersion on the transmission path may be compensated for by using the digital signal processing technology. However, the receiver may be configured to estimate the chromatic dispersion on the transmission path and compensate for the estimated chromatic dispersion. As a method for chromatic dispersion compensation, there is known a method for setting the compensation value for dispersion on the basis of the number of corrections by an error correction circuit after demodulation so as to minimize the number of corrections (see Japanese Laid-Open Patent Application Publication Nos. 2002-208892, 2004-236097 and 2008-58610). There is known another method for optimizing the compensation value for chromatic dispersion on the basis of clocks reproduced by an analog clock reproducing circuit (see Japanese Laid-Open Patent Application Publication No. 2007-60583).